The present invention relates to a novel photosensitive polyimide precursor composition. More particularly, the present invention is concerned with a novel photosensitive polyimide precursor composition which can be used in forming protective films for semiconductor devices and insulating films for multilayer interconnection substrates.
Recently, in the field of electronic industry, polyimides prepared by heat-treating poly(amic acid)s as polyimide precursors have been used as protective films and interlayer insulating films because of their superior heat resistance and dielectric characteristics. In the case of using a poly(amic acid) of a conventional structure, however, a complicated process using a photoresist is required for forming a pattern on a polyimide film obtained from such poly(amic acid). In order to solve this problem there has been proposed the use of a photosensitive polyimide precursor which permits direct formation of a pattern through exposure and development.
As such photosensitive polyimide precursor, in U.S. Pat. No. 3,957,512 there is proposed one wherein a photosensitive group is ester-linked to the carboxyl group in a poly(amic acid) molecule. This photosensitive polyimide precursor is prepared by reacting a tetracarboxylic acid dianhydride with an alcohol containing carbon-carbon unsaturation having photoreactivity to form a tetracarboxylic diester, then converting free carboxyl groups in the diester into acid chloride groups, and polycondensing the resulting diester-bis-acid chloride with a diamine. However, in the photosensitive polyimide precursor obtained by this process there eventually remains a chloride as a by-product because a dehydrochlorination reaction is included in the manufacturing process. For removing this residual by-product chloride which is harmful, it is necessary to conduct a purification treatment which is expensive.
In U.S. Pat. No. 4,654,415 there is disclosed a photosensitive polyimide precursor wherein a photosensitive group is ester-linked to the carboxyl group in a poly(amic acid) molecule, using a process different from the process just referred to above. This photosensitive polyimide precursor is prepared by reacting a tetracarboxylic dianhydride with an a alcohol containing carbon-carbon unsaturation having photoreactivity to form a tetracarboxylic diester and then reacting the diester with a diamine in the presence of carbodiimide. Also in this process, however, there remains a urea derivative as a by-product and a purifying treatment for separating the urea derivative is required in preparing a coating solution of the photosensitive polyimide precursor.
In U.S. Pat. No. 4,243,743 there is disclosed a photosensitive polyimide precursor prepared by incorporating in a poly(amic acid) a compound containing carbon-carbon unsaturation having photoreactivity and also containing an amino group or a quaternized salt thereof, allowing a photosensitive group to be salt-linked to the carboxyl group or a salt thereof contained in the poly(amic acid) molecule. This photosensitive polyimide precursor can be prepared without the formation of any harmful by-product. For example, however, in the case of using a polyimide film as a semiconductor protecting film, the larger the film thickness, the greater the .alpha.-ray shielding effect. When the surface of film formed from the said photosensitive polyimide precursor is masked for patterning, followed by exposure and development using a developing solution and subsequent heat-treatment to obtain a pattern of the polyimide film, it is very difficult to obtain a polyimide film pattern having a thickness of 10 .mu.m or more. This is for the following reason. In the photosensitive polyimide precursor, usually a photosensitive group is introduced into the poly(amic acid) which has a high molecular weight and it is introduced by such a relatively weak linkage as salt linkage, so in the case of developing a thick film, the dissolving speed of an unexposed portion into the developing solution is low, and until a complete dissolution of the unexposed portion into the developing solution, the exposed portion is also dissolved in a considerable amount in the developing solution, resulting in that the thickness of the residual exposed portion becomes smaller.
The present invention has been accomplished in view of the above-mentioned drawbacks of the prior art, and it is the object of the invention to provide a photosensitive polyimide precursor composition which can be prepared without the formation of any harmful by-product requiring the application of a purifying treatment and in which, at the time of development after exposure, the amount of an exposed portion dissolved into a developing solution until a complete dissolution of an unexposed portion in the same solution is small, thus affording a pattern of a polyimide film having a large thickness.